We introduce the notion of a Real Equation System (RES), which lifts Boolean Equation Systems (BESs) to the domain of extended real numbers. Our RESs allow arbitrary nesting of least and greatest fixed-point operators. We show that each RES can be rewritten into an equivalent RES in normal form. These normal forms provide the basis for a complete procedure to solve RESs. This employs the elimination of the fixed-point variable at the left side of an equation from its right-hand side, combined with a technique often referred to as Gauß-elimination. We illustrate how this framework can be used to verify quantitative modal formulas with alternating fixed-point operators interpreted over probabilistic labelled transition systems.
@InProceedings{groote_et_al:LIPIcs.CONCUR.2023.28, author = {Groote, Jan Friso and Willemse, Tim A. C.}, title = {{Real Equation Systems with Alternating Fixed-Points}}, booktitle = {34th International Conference on Concurrency Theory (CONCUR 2023)}, pages = {28:1--28:17}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-299-0}, ISSN = {1868-8969}, year = {2023}, volume = {279}, editor = {P\'{e}rez, Guillermo A. and Raskin, Jean-Fran\c{c}ois}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2023.28}, URN = {urn:nbn:de:0030-drops-190225}, doi = {10.4230/LIPIcs.CONCUR.2023.28}, annote = {Keywords: Real Equation System, Solution method, Gau{\ss}-elimination, Model checking, Quantitative modal mu-calculus} }
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